911 hard starting cold

[zimmermanm]

I am fixing a 1980 911 3.0 ltr sc for a friend of mine. The car runs and starts great when hot. It is extremely hard to start cold. I checked the fuel pressure and its in spec when hot. However, the control pressure is very high when cold causing it to be too lean. I checked the control pressure regulator and it looks good. Clean and the preset plug has not been moved. The pressure relief on the fuel distributor had an o-ring with a nik in it so I replaced that. Still too high during cold start. Finally I removed the line from the bottom of the accumulator ( there are 2 at the top and one at the bottom). I pinched off the return hose that ``Ts` off the bottom and the car started right up and the control pressure is normal. The other thing I noticed is that fuel is coming out the bottom of the accumulator. Is this normal or is the accumulator diaphram torn. Thanks for any help with this before I order a new accumulator. One more thing the fuel pressure when shut off never held pressure more than 5 minutes or so.
Mike.

[ossiblue]

I'd say it's the accumulator as, 1) the system should hold pressure for considerably longer than five minutes, and 2) the accumulator should not leak fuel. Not an expert on this, but having a CIS and following the many posts regarding problems like yours, I belive the accumulator may be faulty.

[boyt911sc]

Mike,

There is always fuel from the bottom of the fuel accumulator. Why???? The bottom port is connected to the main fuel line return to the tank. And everytime the system is pressurized (FP running), the bottom chamber of the fuel accumulator gets fuel too. You need to pressurize the upper chamber to check for the diaphragm. In short, you need to determine whether the fuel coming from the bottom chamber of the FA (fuel accumulator) is coming from the top (delivery) via the diaphragm or not.

Do not replace suspected CIS component/s unless verified defective. Each CIS component could be bench tested conveviently. So to save time and money, avoid guess work in CIS troubleshooting. HTH.

Tony

[boyt911sc]

Mike,

While your FA maybe the culprit causing you to lose the residual pressure and resulting to the starting problem you're experiencing, I need to ask you something. What were the cold control and system pressures? The WUR maybe contributing to your problem too. Just curious. Thanks.

Tony

[spuggy]

I thought the fuel accumulator was mostly to prevent hard starting when *hot* by maintaining pressure in the lines to the injectors and thus prevent the fuel boiling and vapor locking?

Spec booklet says you should still have "at least" 1.3 bar after 10 minutes when hot, 1.1 bar after 20 minutes for all variants of an '80 SC....

[Paulporsche]

You've already told us the cold cp is too high, resulting in the mix being too lean @ startup. Most likely that's the problem. You don't say where you are, but if you search or check the Bentley, there is a chart for your WUR/year/ambient temp. If you try knocking the plug to get the ccp back in range, that will probably fix it.

[zimmermanm]

Paul. Thanks for the reply. I tried that and it did not change at all. I reset the plug and started looking for return line blockages. However, I have yet to find why the return is causing a high cold control pressure.
Mike.

[ossiblue]

When you pinched off the fuel return from the accumulator, you said the car started and ran fine when cold. Did you happen to measure the pressure when you did that?
This is just a wild guess and others are the experts but, if the accumulator is bad could that cause essentially an "open loop" which by-passes the WUR and doesn't allow it to reduce control pressure? By closing off the loop (pinching the return line from the accumulator) you allow only the WUR to control the fuel through the system, thus proper pressure when cold.
Could this be happening? Just a theory.

[zimmermanm]

I removed the return line that goes to the bottom of the accumulator and put it in a container. I started the car and the control pressure is still at 60 psi and it runs rough. There is a blockage some where or the warm up regulator is not working properly? I am going to check the line from the regulator to the fuel ditributor for blockage and if thats not it I will dismantle and inspect the warm up regulator again. Thanks for the help guys and any other idaes are welcome.
Mike.

[ossiblue]

Sounds more and more like the WUR since your last test. You mentioned the word "clean" in your first post but it's not clear what you meant. There is a screen in the connections to the WUR that can get clogged and that may be your issue. Simple first step to check.

[Gunter]

There is a small screen in the top nipple of the WUR that can get clogged.
US 1980 SC has a stand-alone WUR: Bosch 0 438 140 072; check the number.
You can make it adjustable.

IMHO, the bottom line of the Accumulator should not have any fuel in it. I believe that the line was added to drain fuel via the return line if the diaphragm leaks.
Since you say it doesn't hold pressure for long, it may be defective.

[boyt911sc]

Originally Posted by Gunter

There is a small screen in the top nipple of the WUR that can get clogged.
US 1980 SC has a stand-alone WUR: Bosch 0 438 140 072; check the number.
You can make it adjustable.

IMHO, the bottom line of the Accumulator should not have any fuel in it. I believe that the line was added to drain fuel via the return line if the diaphragm leaks.
Since you say it doesn't hold pressure for long, it may be defective.

Gunter,

This sounds like a broken record........and the subject has been discussed several times recently. The three (3) port fuel accumulator will always have fuel in the lower chamber once the system is pressurized (FP running). WHY????? The bottom port (drain tube) is connected to the main fuel return line. Whether the FA is good or bad, the lower chamber will be filled with fuel once the system is pressurized. The excess fuel from the distributor will return to the tank and subsequently fill the lower chamber with fuel too. This is just basic fluid flow dynamics.


Mike,

You need to disconnect the return line of the WUR and determine where the blockage is coming from. It could be internal (WUR) or the outside the WUR. Like a pinched or restricted fuel line.

The 60 psi cold control pressure (from PM) is too high even for WARM control pressure. You won't be able to start the motor with this kind of cold control pressure.

Before you condem the WUR, do some pressure checking before and after the WUR unit. Are you sure that the return line is not restricted? Keep us posted.

Tony

[Gunter]

Good stuff.

Several reasons why I am sceptical about the bottom line normally having fuel in it:

My 3-line FA has no fuel in the bottom line connected to the return line, it's dry!
Plus, there were years when there was no bottom line on a FA. (1974-77?79?) and the system worked.

Also, any pressure coming from the return line would counter-act on the diaphragm in the FA which is spring-loaded AFAIK and compressed by the top line which has high pressure from the pump.
What would be the sense in having pressure coming from the bottom line??

After searching German, UK and US sites for info on Bosch FA, nobody mentions that a bottom line should have fuel in it.

The cold pressure obtained is obviously wrong and may not have anything to do with the FA.

[MatthewBrum]

I would take the WUR apart, clean it, and press both sides up so you can adjust the hot & cold CP by tapping the plugs back down. I have had problems going by the specs in the Bently manual for my 80SC. I find that a cold CP of 45PSIG is about as low as I can go. This is when the warm CP is set to 55PSIG.

You also have to make sure the frequency valve has a duty cycle of 60% while warming up.

[zimmermanm]

Thanks again for the input guys! I was looking at the picture that was posted of a cross section of the WUR and the arm looks bent down, mine is straight and always has pressure from the spring pushing on the pin. It could be that the arm does not relief the pressure on the metal diaphram inside. I am going to take it apart again and really look it over.
Mike.

[ossiblue]

Be careful. There were other posts regarding the arm and one person did, in fact, bend the arm slightly and it worked. However, the amount of travel the arm makes and the distance it pushes on the pin is very small anyway so proceed with caution. Have you checked to see if the heating element is working?

Just for reference. My car had the exact same symptoms with the lack of fuel pressure changes and the inability to hold pressure. My mechanic had to rebuild the WUR after the easy steps (cleaning the screen, looking for obstructions) did not solve the problem. The heating element needed replacement, the 0-ring in the diaphram cavity needed replacement, the diaphram unit needed adjustment within the housing, the pin holding the arm needing positioning, and other things that only he figured out. Bottom line, it was the WUR and your problem is identical. Could be other factors too, but at some point the WUR needs close inspection.

[1982911SCTarga]

Gunter, I believe Tony's correct. When the system is pressurized on a three-port accumulator, there will be fuel in the lower port return line and the bottom portion of the accumulator.

Brian

[zimmermanm]

Hey guys found the problem!!!! I took the wur apart and cleaned it so it was perfect. I tried adjusting the movable plug again with no luck. I then removed the return line from the wur to the back of the fuel distributor and put it in a container. It had fuel coming out when running but the pressure was still at 60psi. I had an extra fuel injection line from a vw system so I put it on the wur return into a can and the presure went to 20 psi!!! I hooked up the new line to the fuel distributor and presto it had normal pressure. I set up the movable plug and ran it, no more stalling and backfiring. The line I removed was making a hissing sound for about a minute so there is an internal plug somewhere. I still suspect the accumulator as the pressure leaks down to 0 psi withing 20 minutes. These old cars can sure get you thinking!!! Now to find a good line. Thanks for all the suggestions, just goes to show you a big problem can be a simple cure.
Mike.

[boyt911sc]

Originally Posted by 1982911SCTarga

Gunter, I believe Tony's correct. When the system is pressurized on a three-port accumulator, there will be fuel in the lower port return line and the bottom portion of the accumulator.

Brian

Brian,

People has the right to express their opinion. But when it comes to technical information, opinion or guess work is superseded by reliable test data. This test about three-port fuel accumulator was performed and observed numerous times. I have first hand experience and knowledge about the fuel line return configuration and its function.

Our friend from the north of the border is not that easy to convince. It took me almost three (3) years to finally make him realized that the TTS (thermotime switch) does not supply POWER to the CSV (cold start valve) during initial start-up. It was a ground source to complete the circuit. The wiring diagrams obviously were not sufficient to change his opinion about the topic. I believe he has finally conceded for the meantime.

Anyway, I believe this change of ideas is not to be discouraged. The facts will eventually show which side of the story is correct. The goal is find the correct information for everyone to share in his/her endeavour.

Thanks Brian for sharing the same belief.

Tony

[Gunter]

Originally Posted by 1982911SCTarga

Gunter, I believe Tony's correct. When the system is pressurized on a three-port accumulator, there will be fuel in the lower port return line and the bottom portion of the accumulator.

Brian

Don't mind being corrected if it makes sense.

Tony, you da man, thanks for hanging in there for 3 years, LOL

Statemts are easily made, but, I love to see an official source for why the bottom portion of the FA needs to be pressurized, preferably from Bosch or Porsche.

So far, various searches only confirmed that the spring-loaded diaphragm should not leak; if it does, the FA is defective.

The FA is supposed to hold pressure, not leak it through the diaphragm and the bottom line.

Jim Williams has an excellent write-up about CIS and his description of the Accumulator makes no mention of a pressurized bottom line.

So, where is the credible source to back up the statement?
Please, point to some Tech source.

Much appreciated.